The invention relates to a process for the production of highly compressed paper sheets comprising celluloses with the addition of thermoplastic synthetic fibers with a volume weight .gtoreq. 0.9 kg/dm.sup.3, as well as the use of the same.
It is already known to manufacture laminar structures through the partial or complete use of thermoplastic synthetic fibers. Through the addition of synthetic fibers to the cellulose, for example, modifications in the strength characteristics or in the surface properties of the laminar materials, designated in the following as the paper sheet or paper in the broader sense, are sought as the objective.
Polyamide, polyethylene, polyester or polypropylene fibers, for example, the melting temperatures of which frequently are more or less distinctly above the surface temperatures (convection, IR, other contact-free drying processes) of approximately 85.degree. C. to 130.degree. C. which are usual in the manufacture of paper, are of consideration as synthetic fibers. They often represent only one type of reinforcing or support material with a strength improving effect in the formed cellulose sheet, but then do not bond irreversibly through thermal diffusion with the cellulose fibers at the intersecting points.
It is known from the manufacture of non-woven fabrics, for example, that this laminar structure results from loose fiber through thermal hardening treatment or precise thermo diffusion by means of so called binding fibers. The fiber which forms the nonwoven fabric is thereby designated as the support fiber, and the melting component is designated as the binding fiber. These binding fibers are divided into the 3 primary groups:
Adhesion fibers; PA0 Bicomponent fibers; and PA0 Thermoplastic adhesive fibers. PA0 123.degree. C. in the case of LLDPE (linear low density polyethylene) fibers; PA0 132.degree. C. in the case of HDPE (high-density polyethylene) fibers; PA0 160.degree. C. in the case of acrylic fibers (copolymers of acrylonitrile and methyl methacrylate).
Adhesion fibers, for example, are non-stretched, amorphous polyester fibers, which soften on the surface at barely 100.degree. C., and thereby become sticky and capable of bonding. A complete calendaring is necessary for this. The necessary proportion of these very expensive adhesive fibers to the total portion of fibers is relatively high so that their purpose of application is limited, for example, to non-woven fiber materials or electrical insulation.
The most elegant solution for the thermal hardening is attained with the bicomponent fiber (mostly core-casing fibers with low-melting casing polymer as the adhesive component). In order to attain an adequate thermofusion with other fibers, however, high additions of these bicomponent fibers are necessary. The use of these is therefore only justified in the manufacture of non-woven fiber material of the highest valve.
On the other hand, many practical cases of application and areas of use can be covered by means of thermoplastic adhesive fibers. In principle, every thermoplastic fiber with a melting range from approximately 100.degree. C can be used as a thermoplastic adhesive fiber. The ideal thermoplastic adhesive fiber should first begin to soften and deform before reaching the melting temperature. Depending on the type of thermoplastic fibers which are used, the melting temperature generally lies between:
120-140.degree. C. in the case of copolyamide fibers;
145-175.degree. C. in the case of copolyester fibers;
215-218.degree. C. in the case of polyamide fibers;
245-260.degree. C. in the case of polyester fibers; and
Under the supposition that an addition of thermoplastic synthetic fibers to conventional cellulose fibers, even during the production or finishing of the paper (for example, off-line glaze finishing), and the temperatures which thereby arise, should lead through thermofusion to irreversible contacts at the intersecting points between the natural cellulose and the synthetic thermoplastic adhesive fibers, the multiplicity of types of usable thermoplastic adhesive fibers is reduced to such as have a crystal melting point below 200.degree. C, preferably below 150.degree. C. It is thereby assumed that the softening range of these thermoplastic synthetic fibers is generally lower for example, with PE-homopolymers (HDPE) it is from 95.degree. C., and in the case of PE-copolymers (LLDPE) it is from 72.degree. C.
The use of synthetic fibers during the production of special papers is already known from the patent and specialized technical literature. The first of these involve, for example, oriented polyethylene fibers which are used for the substitution of asbestos in reinforced cement, resin or floor materials (EP 0292 285 Al), and multiple-layer structures with one or more layers of synthetic fibers (polyethylene terephthalate-copolymer with cellulose with melting points of 110.degree. C), combined with cellulose sheets for agricultural products (EP 0255 690 AI), or combinations of vegetable fibers (wood chips, among others) and polyolefins (polypropylene), which are deformed into foil-like materials by means of hot calendering at temperatures of between -72 and I90.degree. C. In this, value is always placed on the most voluminous possible surface structure with opacity which is thereby higher.
Indications are likewise to be drawn from the specialized technical literature regarding the partial addition of synthetic fibers to the cellulose, such as for example, the use of polymer powders of unstated chemical composition for the production of washable wallpapers of the highest possible porosity and opacity, whereby the laminar structure has also been subjected to a hot calendering (Cellulose Chemistry and Technology [l98I], number 15, pages 125-132).
In another technical publication (Paper Technology and Industry [1979], number 1/2, pages 32-34), the addition of up to 70% synthetic fibers of polyethylene ("Hostapulp" by Hoechst) in a layer is recommended for the production of two-layer imprinted or peelable wallpapers of 150 g/m, The fusion of the synthetic fibers with the cellulose is carried out by supplying hot air (135-170.degree. ) and/or by means of hot calendering (140.degree. C.). Through this means, too, the highest possible opacity is additionally sought.
The use of u to 100% polyethylene fibers in the two covering layers of three-layer laminar composite paper structures, as well as the fusion of these by means of irradiation heat (IR preheating up to 37-54.degree. C. sheet temperature), and the subsequent glaze finishing at ambient temperature, is described in the journal Tappi (1985), number 7, pages 94-97.
The goal of the invention was the development of multiple layer laminar paper sheets from polyethylene and cellulose as an alternative to paper sheets with good barrier characteristics which are laminated with polyethylene foil or extruded polyethylene. The maintenance of the high level of opacity of these triplex papers which has been sought, but which had more or less decreased because of the selected conditions of the thermofusion, presented difficulties. Such triplex papers with polyethylene cover layers are recommended as alternatives for the known polyethylene layered papers (which are mostly polyethylene-extruded), and also as detachable backing papers, among others.
The addition of up to 20% synthetic fibers from polyolefins (polyethylene, polypropylene) to the cellulose in order to attain both high opacities and good printing characteristics after the coating of the sheet with combinations of pigment bonding agent, is recommended in Tappi (1985), number 10, pages 91-93.
The influence of a moist-hot glaze finishing on paper sheets with the addition of synthetic fibers is discussed in Paper Technology and Industry (1975), number 10, pages 309-312. The addition of HDPE fibers to the cellulose thereby amounted to between 0 and 90%.
It was the goal of the latter invention to find in papers with addition of synthetic fiber and within a range of the surface-covered mass of 50-60 g/m.sup.2, glaze finishing conditions which provided both high volume (slight volume weight) and high opacity along with simultaneously improved smoothness to the paper. It was found that the improvement of the smoothness was proportional to the increase of the volume weight. Because papers with an addition of synthetic fibers have a higher density than pure cellulose papers, it was possible to achieve increases in smoothness with simultaneously high paper volume and good opacity by means of moist-hot glaze finishing (20-80.degree. C., 3-9% moisture before the glaze finishing, 35-350 kN/m pressure).
Upon attaining the so-called critical density (volume weight) of the paper of 60 g/m.sup.3 which, depending on the portion of synthetic fiber which lies between 0.6 kg/dm.sup.3 (90% synthetic fibers) and &lt;0.9 kg/dm.sup.3 (0% synthetic fiber), there resulted an undesirable dramatic reduction in the opacity and an increasing blackening of the paper surface which was connected with the formation of transparent spots when using steel-to-rubber rollers. The authors therefore recommend calendering conditions which only effect a compression below the critical paper density. With a 20% synthetic fiber portion in the paper (60 g/m,) for example, the critical paper density of &lt;0.8 kg/dm.sup.3 would be surpassed by means of steel-to-rubber rollers. On the other hand, however, a steam moistening of the paper (superficial application of water) makes high surface smoothness possible, with minimal loss of opacity. In the technical information sheets of the manufacturer of polyethylene fibers, a critical density of the papers of 0.65 kg/dm.sup.3 (steel/steel) or 0.70 kg/dm.sup.3 (cotton/steel rollers) is stated. Under such types of optimized calendering conditions on the large-scale technical level, opacities of approximately 88% at 60 g/m.sup.3 paper (surface-pigmented) were obtained.
The task which forms the basis of the invention is, on the other hand, that of creating a foil-like material from cellulose and synthetic fibers which has a gross density equal to or greater than the critical range, that is to say 0.9 kg/dm.sup.2, and thereby a transparency of 35%. The transparency is necessary because a control of the photocells in the technical processing processes thereby becomes possible, for example, in labelling processes. This task is resolved by means of the process measures stated in the claims, as well as by the applications stated.
The sheet material may also include sizing, retention and wetting agents and fillers.
In contrast with the highly compressed silicon backing papers which were previously known, the paper in accordance with the invention has better tightness against solvents, higher dimensional stability, lower water absorption relative to the influence of moisture, lower porosity, and greater smoothness/lower microcoarseness. With the addition of synthetic fiber, the paper in accordance with the invention occupies in terms of its characteristics a middle position between a classical silicon backing paper of -00% cellulose and the foils of polyethylene (LLPE or HDPE), polyester, oriented polypropylene or polystyrol likewise used for the silicon coating. Although foils are more expensive than papers, they are preferred and used specifically where high transparency, toughness, barrier characteristics or heat-sealing capabilities are desired. Furthermore, because of their closed surface, foils now require smaller application quantities of silicon resins of approximately 50% in order to attain the same level of separation force as siliconized papers.
The papers in accordance with the invention with the addition of synthetic fibers to the polyethylene basis has along with a lesser need for silicon, better rigidity, and above all, higher temperature resistance in comparison with foils. It is precisely that the drying temperature after silicon coating is limited by the possible thermal deformation in the case of polyethylene foils as well as foils of polyester and polypropylene.
During the silicon coating of paper, drying temperatures between I50 and 220.degree. C are conventional. In the case of foil coatings, drying temperatures which are approximately 30-50% lower, and thus the hardening time must be taken into account as well. The manufacture and the characteristics of the transparent paper in accordance with the invention with the addition of synthetic fiber onto polyethylene base will be illustrated in greater detail in the following examples of execution.